My goal is to investigate the structure and function relationships of the trimeric GTP-binding proteins. The model structures of these trimers were built using the coordinates of the crystal structure of transducin alpha-beta-gamma trimer. The following aspects of the G-protein function were modeled: the residues responsible for the GDP/GTP exchange that leads to activation of the alpha subunit; the solvent accessible surface changes upon GTP/GDP exchange, by comparing the crystal structures in GTP and GDP forms; the contact surface changes related to binding of beta-gamma subunits, the receptor and the effector. The surface changes indicated by modeling are being studied by scanning mutagenesis. The modeling showed that the A336S mutation in Gs, found in two types of endocrine diseases, acts via disrupting the binding of the guanine ring. The models also helped in explaining the effects of the N-terminal and C-terminal mutations in the alpha subunit.